1. Field of the Invention
The present invention relates to improved pharmaceutical formulations comprising liposomes incorporating aromatic polyene antibiotics, and the use of these formulations in the treatment of fungal disease. Particular aromatic polyene antifungals which are advantageously employed in the practice of the invention include candicidin and hamycin, both having aromatic moieties such as p-aminoacetophenone incorporated into particular polyene heptaene macrolide structures. The preparation of aromatic polyene antibiotic-containing liposomes suitable for parenteral administration is disclosed.
2. Description of the Related Art
The polyenes, in general, are macrolide antibiotics that have selectivity for inhibiting organisms whose membranes contain sterols. It is believed that their activity is due to the binding of the drug to cell membrane sterols, resulting in a change in membrane permeability. This altered membrane structure permits the leakage of small molecules, and thus disrupts cellular homeostatic mechanisms (1,2). Moreover, in that the polyenes likely function by sterol binding, it is not surprising that microorganism resistance to polyene action is typically a function of its binding affinity for the particular sterol of the microorganism. Typical uses for conventional polyenes such as nystatin and amphotericin B include treatment of fungal diseases such as those resulting from Candida infections (e.g., C. albicans and C. tropicalis) as well as a variety of other diseases such as histoplasmosis, coccidioidomycosis, systemic sporotrichosis, aspergillosis, mucormycosis, chromablastomycosis, blastomycosis and cryptococcosis (3,4).
The foregoing diseases, in general, are often characterized as "opportunistic infections" in that their incidence tends to coincide with one or more "opportunistic" changes with physical state of the host. For example, the incidence of fungal infections tends to dramatically increase in the case of the immunocompromised individual, such as a patient having AIDS or undergoing cancer treatment. Thus, the polyene antibiotics have become important adjuncts in the treatment of certain opportunistic infections in individuals afflicted with AIDS or cancer. Unfortunately, the generally poor physical state of AIDS patients and others seriously ill with fungal related infections, coupled with the serious toxicities associated with macrolide polyene use, renders the use of drugs such as amphotericin B in these patients difficult at best to manage.
Although amphotericin B formulations, as well as those other non-aromatic polyene macrolides, remain of principal importance in the management of fungal disease, their overall usefulness is typically limited by the appearance of untoward effects often associated with systemic application. For example, upon parenteral administration using conventional formulations a large number and variety of untoward effects may be associated with amphotericin B is use. These toxicities include a range of symptoms, from the appearance of hypersensitivity reactions such as anaphylaxis or convulsions on occasion, to irritative and toxic effects, even acute hepatic failure (4). In general, the major cause for concern is due to the occurrence of nephrotoxicity: over 80% of persons given amphotericin B develop decreased renal function and abnormal urine sediment, evidence of toxic hepatic cell degeneration (4).
Coupled with the toxicity problems of polyene macrolide therapy is the very high degree of failure in immunosuppressed patients (5). This failure is thought to be mainly due to the immunosuppressed state of the patient, as well as the inability to deliver higher quantities of the drug. In short, the polyene macrolides typically exhibit a very poor therapeutic index or "margin of safety" when administered parenterally in the treatment of fungal disease.
The "therapeutic index" of a drug is intended to represent the ratio of the upper dosage limit of a drug that will give a certain level of toxicity (e.g., LD.sub.50, maximum tolerated dose MTD), over some minimum level of biologic activity (e.g., minimum inhibitory concentration, MIC). The polyenes, unfortunately, tend to exhibit toxicities at levels not far removed from therapeutic levels, and this limitation represents an important problem.
One class of polyene antibiotics which has shown considerable antifungal activity in in vitro susceptibility tests are polyenes whose structure include aromatic groups (5-9). These aromatic-containing polyenes are exemplified by agents such as hamycin, candicidin, aurefungin, ascosin, ayfattin, azacolutin, DJ400-B, trichomycin, levorin, heptamycin, candimycin and perimycin, whose structures include aromatic groups such as p-aminoacetophenone or N-methyl-p-aminoacetophenone, which are attached to a macrolide backbone (6). Unfortunately, although these agents are highly active in vitro, they have yet to find utility in the clinical management of disease due to both their highly toxic nature as well as their poor solubility. In short, although the aromatic polyene antibiotic exhibit very high and broad spectrum antifungal activity (e.g., measured in terms of MICs against various fungal species), even at very low doses of activity, these agents tend to be toxic. In addition, the aromatic polyene antibiotics present other particular problems in terms of water insolubility, which further renders their use difficult and impractical.
The polyene antifungals are the most potent antifungal antibiotics. Amphotericin B is the only commercially available parenteral polyene and is the most important drug for the treatment of serious fungal infections (1). This drug is very toxic and is unlikely to cure fungal infections in severely immunosuppressed patients (2). Candicidin is a polyene antifungal available only for the topical treatment of superficial fungal infections. No parenteral formulation is available for the therapy of deep seated mycoses, because the drug is totally water insoluble and very toxic. Candicidin however has an aromatic moiety that is believed to confer to this antibiotic a much better antifungal activity than amphotericin B or other polyenes that lack this aromatic moiety in their structure. Many in vitro susceptibility experiments conducted in various laboratories, including those of the present inventors, indicate that candicidin has a much higher antifungal activity than amphotericin B against various fungi. It is therefore believed, that if a safe parenteral formulation of candicidin would be available, many of the therapeutic failures with amphotericin B could be avoided. Of note, candicidin has been given orally (without any toxicity) to humans in this country.
Accordingly, there is currently a need for improved pharmaceutical formulations particularly suited for the treatment of fungal disease. These formulations should address one or more of the disadvantages associated with previous formulations. There is, in particular, the need for formulations which exhibit broader therapeutic indexes, including lower minimum effective doses, or higher toxic doses, or both. Moreover, there is a need for safe, parenteral formulations having a sufficiently broad spectrum and/or high degree of efficacy of antifungal activity. These properties would render them particularly desirable for use in treatment of immuno-compromised individuals.
It has recently been shown that the encapsulation of certain drugs in liposomes before administration to the patient can markedly alter the pharmacokinetics, tissue distribution, metabolism and therapeutic efficacy of these compounds. Liposomes may be defined as lipid vesicles which are formed spontaneously on addition of an aqueous solution to a dry lipid film. Further, the distribution and pharmacokinetics of these drugs can be modified by altering the lipid composition, size, charge and membrane fluidity of the liposome in which they are encapsulated.
Liposomes have been used as carriers of amphotericin B for treatment of murine leishmaniasis, histoplasmosis, cryptococosis and candidiasis. Liposome-encapsulated amphotericin B has also been used for treatment of coccidioidomycosis in the Japanese macaque.
It has been demonstrated that liposome-encapsulated amphotericin B (AmpB) may be used to treat experimental murine candidiasis and in the treatment of fungal infections in patients with leukemia and lymphoma.